1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus which applies a radiofrequency magnetic field and a gradient magnetic field to a body placed in an uniform magnetostatic field according to a predetermined pulse sequence, and detects the magnetic resonance signal from the body to be tested to image the internal information of the body to be tested as a magnetic resonance image, and a magnetic resonance image correction system used for the apparatus.
2. Description of the Related Art
The magnetic resonance imaging method is, as well known, a method to form an image of the chemical and physical microscopic properties of materials, by utilizing a phenomenon which resonantly absorbs the energy in the radiofrequency magnetic field which rotates at a specific frequency, when a group of nuclear spins having an inherent magnetic moment is located in an uniform magnetostatic field.
In this magnetic resonance imaging method, there can be obtained images having various contrasts, such as an image with a contrast in which a longitudinal relaxation time T.sub.1 of the nuclear spin is emphasized (hereinafter referred to as "T.sub.1 image"), an image with a contrast in which a transverse relaxation time T.sub.2 of the nuclear spin is emphasized (hereinafter referred to as "T.sub.2 image"), an image with a contrast in which the density distribution of the nuclear spin is emphasized (hereinafter referred to as "density image"), an image with a contrast in which a transverse relaxation time T.sub.2 of the nuclear spin and an actual transverse relaxation time T.sub.2 * reflecting an abrupt phase change of the nuclear spin due to the microscopic nonuniformity of the magnetic field in the voxel are emphasized (hereinafter referred to as "T.sub.2 * image"), and the like.
As described in "Perfusion Imaging" (John A. Detre et al., Magnetic Resonance in Medicine, 23, pp.37-45 (1992)), when an amount of local bloodstream or a flow rate of bloodstream in the living body tissue changes, in certain imaging methods of the magnetic resonance imaging apparatus, it is observed that the relaxation time (for example, T.sub.1 and the like) of the living body tissues apparently changes, and the image contrast is changed. In these imaging methods, information relating to the perfusion and diffusion in the living body can be effectively obtained by utilizing a contrast medium. In the imaging method with regard to perfusion and diffusion, there are often determined the difference among a plurality of images in order to examine the timewise change, or the adding average in order to detect minute signal change. In this case, however, there is a problem of positional deviation among images due to the movement of a living body and the like.
On the other hand, as described in "Oxygenation-Sensitive Contrast in Magnetic Resonance Image of Rodent Brain at High Magnetic Fields". (Seiji Ogawa et al., Magnetic Resonance in Medicine, 14, pp.68-78 (1990)), it is known that among hemoglobins in the blood in the living body, oxidized hemoglobin contained in a large amount in the arterial blood shows the diamagnetism, and reduced hemoglobin contained in a large amount in the venous blood shows the paramagnetism. And as described in "MRI Susceptometry: Image-Based Measurement of Absolute Susceptibility of MR Contrast Agents and Human Blood", (Robert M. Weiskoff and Suzanne Kiihne, Magnetic Resonance in Medicine, 24, pp.375-383 (1992)), the oxidized hemoglobin which is a diamagnetic substance hardly disturbs local magnetic fields (the difference in the magnetization rate from that of the living body tissue: 0.02 ppm), but the reduced hemoglobin which is a paramagnetic substance has a large difference in the magnetization rate from that of the peripheral tissues (the difference in the magnetization rate from that of the living body tissue.: 0.15 ppm) and locally disturbs the magnetic field, thereby T.sub.2 * is shortened.
By utilizing tile above-mentioned property, changes in the oxygen density and changes in the bloodstream due to the physiological functions, such as activation of cells in the living body tissues, for example, activation of the visual cortex in the cerebral cortex with the stimuli of light can be imaged, which is described in "Dynamic magnetic resonance imaging of human brain activity during primary sensory stimulation", (Kenneth K. Kwong et al., Proc. Natl. Acad. Sci. USA 69, pp.5675-5679 (June 1992) Neurobiology) and the like. As the image-sensing method used in forming their image, there is a method based on a pulse sequence, generally called as a gradient echo method and an echo planar method.
However, the signal change (change in the image contrast) caused due to the physiological functions in the living body obtained by these imaging methods is very small. Therefore, as a method for detecting this small signal change, there have been conventionally used a method to take the difference of images before and after the phenomenon of the physiological functions or a method to carry out the statistical processing. As the statistical data-processing method, there can be mentioned a method using the paired-samplet-test detection method which is described in "Functional Brain Imaging at 1.5T using Conventional Gradient Echo MR Imaging Techniques", (R. T. Constable et al., Magnetic Resonance Imaging, Vol. II. pp.451-459, 1993) or the like. Here, when the statistical processing is carried out, since a plurality of images are required, the time for taking images becomes long. On the other hand, when image subtraction is used, it is necessary to obtain an image with high S/N ratio. Accordingly, both methods are readily affected by the movement of a living body.
As a method to correct the movement of the living body, there is a method to correct the scale and the position of the image per se, i.e., to perform a pattern matching. As a well-known method in this pattern matching, there is a method to utilize the cross-correlation among images. The example in which the magnetic resonance image is corrected by utilizing the cross-correlation of phases is described in "Symmetric Phase-Only Matched Filtering of Fourier-Mellin Transforms for Image Registration and Recognitions, (Qin-sheng Chen et al,, IEEE TRANSACTIONS ON PATTERN ANALYSIS AND MACHINE INTELLIGENCE, VOL. 16 NO.12 pp.1156-1168 (1994)).
Furthermore, though it is well known that a distortion in images is caused when the distribution of the magnetostatic field is not uniform, the distortion in images is particularly conspicuous in the imaging method with the T.sub.2 * contrast which is used for the detection of the Phenomenon of physiological functions, such as the cell activation of the living body described above. The method to correct such distortions in images by using the affine transformation is described in Japanese Patent Application Laid-open No. 7-79943.
When data are processed between a plurality of images in order to form an image of informations of the perfusion, diffusion, cell activation and the like in the living body, If there is any inconsistency of images caused by the deviation in position or the change in scale of images due to the influences of movements of a living body, it is difficult to clearly form the image of informations of perfusion, diffusion, cell activation and the like.
It is well known that the scale and the position of the brain actually change simultaneously with the heartbeat, as described in "Brain Parenchyma Motion: Measurement with Cine Echo-Planar MR Imaging", (Brigitte P. Poncelet et al., Radiology, 185, pp.645-651 (1992)).
As described above, in the conventional method, there has been a problem that various physiological functional informations in the living body cannot be detected correctly due to the influences of movements of the body accompanied with the respiration, heartbeat and the like.